Please refer to FIG. 1. FIG. 1 is a diagram shows a magnetic map of the magnetic north. As shown in FIG. 1, the magnetic north varies with the position. The declination refers to the difference between the magnetic north and the geometric north. For example, the declination at the position 101 is sixty degrees; the declination of the position 102 is forty-five degrees; the declination of the position 103 is thirty degrees; and the declination of the position 104 is ten degrees.
The magnetic north may be provided by a magnetometer. However, what a user asks or requires is the geometric north, not the magnetic north. It is noted that the magnetic heading is the heading relative to the magnetic north, while the geometric heading is the heading relative to the geometric north. Hence, how to generate the geometric heading or how to calibrate the geometric heading according to the magnetic north in order to solve the above-mentioned problem has become an important topic in this field.